With the recent spread of cordless, portable devices such as AV appliances and personal computers, there is an increasing demand that batteries for powering such devices should be smaller, more light-weight, and higher in energy density. In particular, there is large expectation for lithium secondary batteries having high energy density, and their potential market size is also large.
Most of the currently commercially available lithium secondary batteries use LiCoO2 as a positive electrode active material, but Co is expensive. Thus, various alternative positive electrode active materials to LiCoO2 are being studied. Among them, lithium-containing transition metal oxides are being intensively studied.
For example, LiNiO2 having a layer structure is expected to provide a large discharge capacity. However, LiNiO2 deteriorates greatly since its crystal structure changes due to charge/discharge. Hence, in order to stabilize the crystal structure during charge/discharge, adding various elements to LiNiO2 has been proposed. Examples of such additional elements that have been proposed include cobalt, manganese, titanium, and aluminum.
LiNiO2 containing an additional element M can be represented by the general formula: Li[Lix(NiaM1−a)1−x]O2. Such a lithium-containing transition metal oxide can be produced by mixing a transition metal compound containing Ni and M with a lithium compound in a predetermined ratio, and heating the mixture to react the transition metal compound with the lithium compound. As the transition metal compound containing Ni and M, for example, a hydroxide is used. As the lithium compound, lithium carbonate, lithium hydroxide or the like is used.
Since lithium hydroxide is more expensive than lithium carbonate, it is more advantageous in terms of production costs to use lithium carbonate rather than lithium hydroxide. However, while the melting point of lithium hydroxide is 400° C., the melting point of lithium carbonate is 650° C. Hence, the temperature at which the reaction between lithium carbonate and a transition metal compound starts is higher than the temperature at which the reaction between lithium hydroxide and a transition metal compound starts by approximately 200° C.
However, when a transition metal compound of a high Ni content is reacted with a lithium compound, a Ni3+ ion tends to be reduced to a Ni2+ ion and included in the lithium site. This tendency increases as the reaction temperature between the transition metal compound and the lithium compound rises. When Ni is included in the lithium site, the product, i.e., the lithium-containing transition metal oxide, has low crystallinity and low characteristics as an active material.
It is therefore common to use expensive lithium hydroxide as the lithium compound to be reacted with a transition metal compound of a high Ni content (see Japanese Laid-Open Patent Publication No. Hei 11-307094). Although a production method using lithium carbonate has been disclosed, the Ni content of the transition metal oxide to be reacted therewith is low (Japanese Laid-Open Patent Publication No. 2002-110167).